The present invention relates to an optical compensating film that drastically improves contrast, display colors and other visual properties of a liquid crystal display (hereinafter this term can be abbreviated as LCD) and also having higher durability and excellent handling properties in a liquid crystal display, an elliptically polarizing plate, and a liquid crystal display using the optical compensating film and the elliptically polarizing plate.
Liquid crystal displays have been used for desk-top electronic calculators, electronic clocks, personal computers, word processors, and meters of automobiles and other machines, and such LCDs comprise polarizing plates. A polarizing plate is produced, for example, by laminating protective films such as triacetylcellulose on both surfaces of a polarizing film of a polyvinyl alcohol-based (PVA) film in which iodine or a dichroic dyestuff is adsorb-oriented. Recently, such films have been required to have a high polarization degree and high transmittance.
A polarizing film can be produced, for example, by adsorbing either iodine or a dichroic dyestuff in a non-oriented polyvinyl alcohol-based film before stretching, or by adsorbing iodine or a dichroic dyestuff in a polyvinyl alcohol-based film that has been stretched and oriented. A polyvinyl alcohol-based film lacks a process stability, and thus it is difficult to stretch uniformly because hydrogen bonds among the molecules are disconnected by moisture during a wet stretch. For producing a film having a high polarization degree and high transmittance, various processes have been proposed. For example, JP-08 (1996)-240715 A describes a process of adsorb-orienting iodine or the like in a polyvinyl alcohol-based film subsequent to a dry uniaxial stretching of the film, and then dipping the film in an aqueous solution containing boric acid at a temperature ranging from 70xc2x0 C. to 85xc2x0 C. JP-10 (1998)-288709 A describes a process of dry uniaxial stretching of a polyvinyl alcohol-based film, dyeing, and further stretching of the film at least 1.5 times in a boron compound. JP-11 (1999)-49878 A describes a process of dry uniaxial stretching of a polyvinyl alcohol-based film, dyeing, and stretching of the film from 1.1 to 1.8 times while dipping in an aqueous solution of boric acid.
However, it is difficult to obtain a film having a high polarization degree in the process described in JP-8-240715 A. In the processes as described in JP-A-10-288709 and JP-11-49878 A, the film becomes susceptible to rupture when the stretch ratio in a processing bath including a boron compound is increased for improving the polarization characteristics. Therefore, a method of obtaining stably a film with a high polarization degree has been desired.
As a result of keen studies, the inventors found that a polarizing film having a high polarization degree and high transmittance is obtainable by uniaxially dry-stretching a polyvinyl alcohol-based film, relaxing the film in an aqueous solution containing either iodine or a dichroic dyestuff, and further stretching the film in an aqueous solution containing a PVA crosslnking agent. An object of the present invention is to provide a polarizing film having a high polarization degree and high transmittance, and a polarizing plate, a polarizing element or a liquid crystal display using the polarizing film.
For achieving the above-described object, a process for producing a polarizing film according to the present invention comprises: uniaxially dry-stretching a polyvinyl alcohol-based film, relaxing the film in an aqueous solution containing iodine or a dichroic dyestuff, and stretching further the relaxed film in an aqueous solution containing a PVA crosslinking agent.
The PVA crosslinking agent is preferably at least one compound selected from the group consisting of boric acid, borax, glyoxal, and glutaraldehyde.
It is preferable in the process that the polyvinyl alcohol-based film has a dry-stretch ratio of at least 2, and the relaxed film has a stretch ratio of at least 1.02. It is also preferable that a relaxation rate of the polyvinyl alcohol-based film is at most 10% of the dry-stretch ratio. Further it is preferable that the polyvinyl alcohol-based film has a total stretch ratio of at most 6.5.
It is preferable in the process that the dry-stretch ratio of the polyvinyl alcohol-based film is from 2 to 6. It is further preferable that the stretch ratio of a relaxed film is from 1.1 to 4.0. It is also preferable that the total stretch ratio of the polyvinyl alcohol-based film is from 4.0 to 6.5.
In the process of the present invention, polyvinyl alcohol-based films having a thickness from 5 xcexcm to 150 xcexcm are preferably used.
It is preferable in the process that the iodine-containing aqueous solution contains iodine in a range from 0.01 to 0.5 weight parts and potassium iodide in a range from 0.05 to 50 weight parts for 100 weight parts of water at a temperature from 10xc2x0 C. to 80xc2x0 C. The aqueous solution containing a PVA crosslinking agent contains a PVA crosslinking agent in a range from 0.1 to 15 weight parts for 100 weight parts of water at a temperature from 10xc2x0 C. to 90xc2x0 C. For the PVA crosslinking agent, a boron compound is used preferably.
In the process, a film stretched in an aqueous solution containing a PVA crosslinking agent can be washed with water and then dried.
A polarizing film according to the present invention can be obtained by uniaxially dry-stretching a polyvinyl alcohol-based film and relaxing it in an aqueous solution containing either iodine or a dichroic dyestuff, and further stretching the film in an aqueous solution containing a PVA crosslinking agent, and the film has a transmittance of at least 43.0% and a polarization degree of at least 98%.
It is preferred that the polyvinyl alcohol-based film has a dry-stretch ratio in a range from 2 to 6, a relaxing rate of at most 10% of the dry-stretch ratio, the relaxed film has a stretch ratio ranging from 1.1 to 4.0 and the polyvinyl alcohol-based film has a total stretch ratio of at most 6.5.
A polarizing plate of the present invention is formed by bonding a protective film on at least one surface of a polarizing film produced in the above-described process according to the present invention.
An optical member of the present invention is formed by laminating either a reflecter or a transflecter on a polarizing film produced in the above-described process according to the present invention.
An optical member of the present invention is formed by laminating a retardation plate or a xcex plate on a polarizing film produced in the above-described process according to the present invention.
An optical member of the present invention is formed by laminating an optical compensating film on a polarizing film produced in the above-described process according to the present invention.
An optical member of the present invention is formed by laminating a brightness enhancement film on a polarizing film produced in the above-described process according to the present invention.
A liquid crystal display of the present invention comprises a liquid crystal cell and a polarizing film that is produced in a process according to the present invention and applied to at least one surface of the liquid crystal cell.